It's not "free" speech when the audience is being coerced into hearing it. Before you say it, yes, the workers are being coerced. The implicit threat is that

It's not "free" speech when the audience is being coerced into hearing it. Before you say it, yes, the workers are being coerced. The implicit threat is that they will lose their jobs if they don't listen to the employer's anti-union rhetoric. "Oh, but they can just quit and not be forced to listen to it," you say? Well, no, the average American worker can't afford to just walk away from a job. Thus, it's coercion.